1. Field of the Invention
This invention relates to a signal processing apparatus for processing 1-bit digital signals obtained by 1-bit sigma-delta conversion, an apparatus for recording the signals and an apparatus for reproducing the signals.
2. Description of the Related Art
The method for recording, reproducing or transmitting digital signals previously converted from analog signals heretofore has been carried out by a recording and/or reproducing apparatus having an optical disc, typified by a compact disc (CD) or a magnetic tape typified by a digital audio tape (DAT), as a recording medium, or has been used in digital broadcasting, such as satellite broadcasting.
In the above-mentioned conventional digital/audio transmission apparatus, for converting the analog audio signals into digital signals, the sampling frequency of 48 kHz or 44.1 kHz or the number of quantization bits of 16 or 20 bits, has been prescribed as formats.
In the above-mentioned conventional digital audio format, audio data is recorded or transmitted simultaneously with sub-data pertinent to the audio data. The copying protection information is recorded in a portion of the sub-data and detected on the reproducing or receiving side by way of performing copying protection.
In the above method for recording the copying protection in a portion of sub-data, these sub-data can be disregarded or re-written for releasing the copying protection. Moreover, audio data remains unaffected by the operation of releasing the copying protection of the above sub-data.
It has also been contemplated to extract part of the band of the audio signals by an acute notch filter and to detect the band extracted by the notch filter in order to perform copying protection. With this method, since the copying protection information is superimposed on and thus included in the audio data, the characteristics of the audio signals are changed by the notch filter if it is attempted to release the copying protection. Thus, even if the function of copying protection is achieved, the sound quality is affected significantly by the processing of copying protection.